Advances from Federal Home Loan Bank available.

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Amount of borrowings as of the balance sheet date from the Federal Home Loan Bank, which are primarily used to cover shortages in the required reserve balance and liquidity shortages.

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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No definition available.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount of the collateral pledged in support of federal home loan bank advances outstanding.

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Amount of advances from Federal Home Loan Bank (FHLBank) maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Other Borrowings and Subordinated Notes and Debentures - Textual (Details) - USD ($)	1 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2018	Jun. 30, 2018	Dec. 31, 2017	Oct. 31, 2017
Short-term Debt [Line Items]
FHLB advances outstanding		$ 945,000,000
Debt Instrument [Line Items]
FHLB advances		1,500,000,000
Additional advances from FHLB		1,500,000,000
Mortgage loans and investment securities securing FHLB advances		3,900,000,000
Assets		$ 16,165,533,000	$ 15,055,806,000
U.S. Bank National Association \| Revolving credit facility
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 75,000,000
London Interbank Offered Rate (LIBOR) \| U.S. Bank National Association \| Revolving credit facility
Debt Instrument [Line Items]
Floating rate (as percent)		1.50%
Federal Home Loan Bank Advances
Short-term Debt [Line Items]
Debt term			1 year
Subordinated debt
Debt Instrument [Line Items]
Subordinated debentures	$ 330,000,000		$ 0
Fixed rate (as percent)	5.00%	5.00%
Debt issuance costs	$ 3,600,000
Subordinated debt \| London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Floating rate (as percent)	2.15%